1. Field of the Invention
This invention relates generally to the field of radio communications systems and more particularly to an improved method and apparatus for measuring the signal strength of a received signal wherein the output signal of the signal strength measuring apparatus is adaptively altered to compensate for the effects of component aging and variations which occur during manufacturing.
2. Description of the Prior Art
In radio communication systems covering larger geographical areas than can be covered with a single radio frequency transmitter, a selection scheme must be used to insure that the transmitter selected will provide good quality communications to the desired remote station.
The problem of selecting which of several possible radio transmitters should be used to respond to a remote station has been solved with a limited degree of success in several different ways. According to one technique, a radio receiving the strongest RF signal from a selected remote station is used to define the approximate location of that remote station. The primary station simply selects the radio transmitter covering the geographical area of the receiver receiving the strongest signals from the selected remote station.
According to another technique, the remote station determines the zone in which it is operating by the transmitter having the largest signal strength. This technique requires that each transmitter in the system operate on an independent frequency, and that communications from the primary station to a selected remote station be sent in all zones in order for the remote station to make its choice known on demand.
Either of the above mentioned techniques requires some method by which signal strength may be accurately measured. Conventional receivers generally derive an analog voltage directly proportional to the received signal by coupling amplification circuitry to the output of the receiver detector circuit. The signal strength indicator signal is then compared to a fixed reference or threshold voltage to generate a squelch signal which is used to turn the receiver audio on or off. A squelch of this type is described in U.S. Pat. No. 4,359,780, to Day, and is assigned to the assignee of the present invention.
Each of the above signal strength techniques and circuitry is subject to variations which occur due to manufacturing tolerances as well as variations which occur in the materials which are used to fabricate the circuitry. Moreover, in a system having a plurality of radios, the receiver sensitivity threshold level may vary at a different rate over temperature, for each respective radio, due to variations in the components used to fabricate the signal strength circuitry.